Conventional electroluminescent devices used in EL displays or EL illumination apparatuses have a layer structure including a cathode, an electroluminescent layer, a transparent electrode (anode) and a transparent substrate. These electroluminescent devices are operated by utilizing such a light emission principle that holes injected from the anode and electrons injected from the cathode are recombined with each other in the electroluminescent layer, so that an emission center therein is excited by the recombination energy to emit a light therefrom.
In the EL displays, although it is required to efficiently extract the emitted light on a side of the transparent substrate, the light extraction efficiency (which means a percentage of light extracted outside from the device to that generated in the electroluminescent layer) on the side of the transparent substrate is as low as about 20% owing to reflection of the emitted light at a boundary surface between the transparent substrate and an outside air layer.
In order to decrease reflection of light at the boundary between the transparent substrate and the air layer, there has been proposed a method of disposing a low-refractive layer between the transparent substrate and the transparent electrode layer to refract light having a large incident angle to substrate from transparent electrode, thereby enhancing the light extraction efficiency (Japanese Patent Application Laid-Open (KOKAI) No. 2002-278477).
In the above conventional method, the reflection of light at the boundary surface between the transparent substrate and the air layer is prevented by providing the low-refractive layer. However, this method has failed to take account of reflection of light at a boundary surface between the transparent electrode layer and the low-refractive layer. Rather, in the method, there tends to be caused such a problem that the provision of the low-refractive layer promotes total reflection of light at the boundary between the transparent electrode layer and the low-refractive layer. The total reflection light thus produced is attenuated upon light guide due to reflection caused inside of the transparent electrode layer and the light-emitting layer as well as at the cathode, and, therefore, cannot be extracted outside from the device.
The present invention has been conducted to overcome the above conventional problems. An object of the present invention is to provide an electroluminescent device in which the total reflection not only at a boundary surface between a transparent substrate and an outside air layer but also at a boundary surface of a transparent electrode layer on its light extraction side is decreased, and which is fully improved light extraction efficiency.